1. Field of the Invention
The present invention relates to a forklift, and more particularly relates to a forklift using an induction motor in order to generate the driving force of a cargo apparatus.
2. Description of the Related Art
In a typical conventional battery forklift, the driving force of the cargo apparatus is generated by a DC motor. For example, Japanese Laid Open Patent Application JP-P 2002-176706A discloses a forklift that uses a DC motor for driving a drive wheel and a DC motor as a steering assistant motor and a cargo apparatus driving motor, in order to improve the convenience and economic efficiency of the forklift. The merit of using the DC motor as the cargo apparatus driving motor is the simplicity of the control of the DC motor.
Recently, a technique using an induction motor controlled in accordance with a vector control for supplying the driving force of the cargo apparatus is proposed. In Japanese Laid Open Patent Application JP-A-Heisei, 7-107797, the vector control of the induction motor is explained.
The induction motor is applied for generating the driving force to the cargo apparatus of the forklift so that the generation efficiency of the driving force is improved. In the battery forklift, the battery is used as a power source. Thus, it is important that the necessary driving force can be generated by the small electric power.
However, the mechanism where the vector control is employed for controlling the induction motor for supplying the driving force of the cargo apparatus brings about the following two problems. First, the vector control is sensitive to the external disturbance. In the control of the cargo apparatus, it is important that the cargo apparatus directly reacts with the operation of a user. For example, when the user operates the cargo apparatus so as to raise a claw of the cargo apparatus, it is important that the claw is quickly raised in response to the operation. However, when the vector control is employed, the directness of the operational feeling is liable to be lost because of the sensitiveness to the external disturbance. For example, when the cargo apparatus is operated so as to raise the claw of the cargo apparatus, in a case where the cargo apparatus has a multiple-step mast, if the stage switching of the mast is carried out, the control system of the induction motor reacts with the stage switching. This drops the responsiveness to the operation of the user of the cargo apparatus. The second problem is the lack of the versatility of the control software of the induction motor. Even if the software of the vector control is prepared so as to match with a certain induction motor, the software cannot be employed for the control of the other induction motor in many cases. This is because the software of the vector control largely depends on the performance parameter of the induction motor. The lack of the versatility of the software is not desired in order to improve the productivity of the forklift.
From the viewpoint of the above mentioned background, the forklift which has the favorable response of the cargo apparatus to the operation of the user and has the high versatility of the control software of the induction motor for driving the cargo apparatus is desired.